The microcapsules are applied to fibres in textile articles known as smart textiles, to impart a controlled release of different products such as fragrances, antibacterial, insecticides, antioxidants, vitamins or durable materials to impart functions, such as thermal insulation and thermal comfort as in the case of microcapsules of PCM (phase change materials). They are also used as special effects materials, as it is the case of photochromic or thermochromic pigments that change colour according to luminosity or temperature, respectively. The binding of microcapsules to the fibres is usually done with thermoplastic binders or with glue (sizing operation). The production of microcapsules of the controlled release type with polymers, is, for example, described in patent GB1371179 of 1974. PCM microcapsules have normally walls made of polymers obtained by the condensation polymerization of urea-formaldehyde and melamine-formaldehyde, given that these materials are very resistant to temperature and to chemical agents and solvents. Other condensation polymers are used, like polyamide and polyurethane, but they are not appropriate for PCM given that they are not sufficiently resistant. They are only appropriate for the release of the active product since they rupture easily. Other microcapsules also for temporary use on products to be used next to the skin are made of biocompatible products such as chitosan, a product obtained from crab or other crustaceous species.
The application of controlled release microcapsules with binders or with glue during sizing textile processing started in the 1970's. The problem with this form of binding microcapsules is that they come off easily during the washing of the textile article or other processes that involve friction forces, given that they do not have a durable bond with the fibre. This way, the desired effect of the microcapsules is quickly lost by wearing the textile article.
It is therefore convenient that the bonds between fibres and microcapsules be resistant to multiple domestic washing, according to the most recent washing standards. The microcapsules for the controlled release of fragrances, antibacterial agents, insect repellents and other active products, are normally applied in such a way so as to be exposed to friction and so rupturing and releasing the products, such as printing with thermoplastic polymers. They can also be applied by glued padding with binders in pad-mangle machines. Normally they are not applied by exhaustion processes given that they have no affinity towards the fibres. Even if they are applied by exhaustion process, the fabric or knitwear still needs to be padded with binders and the microcapsules subsequently fixed by the thermoplastic binder at high temperatures, in appropriate machines, normally a stenter.
PCM (phase change materials) microcapsules on the other hand should not rupture and are normally applied immersed in a coating or foam constituted of thermoplastic polymers. First, the microcapsules are dispersed in a binder and are then bound to the fibres by a thermal process after coating the material with a ruler or rollers. On non-woven it can be done by spraying or by padding followed by thermal fixation in a roller-machine (foulard), always mixed with binders, being one of the corresponding patents from 1994 (U.S. Pat. No. 5,366,801). The thermal process of thermoplastic fusion of the binder containing the microcapsules with the fibres, is usually realized in a continuous drying and curing machine of the type of a stenter used in textile finishing, or under pressure in a heated calendar rollers, at a temperature higher than the melting point of the thermoplastic binder. Given that the quantity of PCM microcapsules is much higher than in the case of the other microcapsules, normally between 30 and 100% of the weight of the fibre, the quantity of binder is also higher. In this case, the durability of the microcapsules is not an issue, since they are totally involved by a film or coating of binder. Phase Change Materials (PCM) are materials that change phase from solid to liquid and from liquid to solid, with the characteristic that and in doing so they absorb great quantities of energy by changing from solid to liquid and releasing great quantities of energy by changing from liquid to solid. Their energy retention characteristics can also be used as a self-regulation of temperature within pre-defined limits, such as, for example, to convey comfort to the wearer of winter clothing and winter footwear. Given that the direct application of PCM microcapsules on yarns, woven fabrics and knitwear present problems, namely technical ones the more usual applications resort to supports, such as polyurethane foam containing PCM microcapsules, or woven or non-woven materials coated with thermoplastic binders containing PCM microcapsules as referred in U.S. Pat. No. 5,851,338. These supports are then incorporated in clothing or footwear articles. They can also be incorporated in composite materials, such as the ones mentioned in U.S. Pat. No. 6,004,662. PCM microcapsules are usually made of polymers, such as urea-formaldehyde or melamine-formaldehyde.